Scholarly article on topic 'Dual Z-scheme charge transfer in TiO2–Ag–Cu2O composite for enhanced photocatalytic hydrogen generation'

Dual Z-scheme charge transfer in TiO2–Ag–Cu2O composite for enhanced photocatalytic hydrogen generation Academic research paper on "Chemical sciences"

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{Z-scheme / "Surface plasmon resonance (SPR)" / "TiO2 " / Photocatalysis / "Hydrogen generation"}

Abstract of research paper on Chemical sciences, author of scientific article — Junwei Fu, Shaowen Cao, Jiaguo Yu

Abstract Photocatalytic hydrogen generation is one of the most promising solutions to convert solar power into green chemical energy. In this work, a multi-component TiO2–Ag–Cu2O composite was obtained through simple impregnation-calcination of Cu2O and subsequent photodeposition of Ag onto electrospun TiO2 nanotubes. The resulting TiO2–Ag–Cu2O photocatalyst exhibits excellent photocatalytic H2 evolution activity due to the synergetic effect of Ag and Cu2O on electrospun TiO2 nanotubes. A dual Z-scheme charge transfer pathway for photocatalytic reactions over TiO2–Ag–Cu2O composite was proposed and discussed. This work provides a prototype for designing Z-scheme photocatalyst with Ag as an electron mediator.

Academic research paper on topic "Dual Z-scheme charge transfer in TiO2–Ag–Cu2O composite for enhanced photocatalytic hydrogen generation"

Accepted Manuscript

Dual Z-scheme charge transfer in TiO2-Ag-Cu2O composite for enhanced photocatalytic hydrogen generation

Junwei Fu, Shaowen Cao, Jiaguo Yu

Materiomics

PII: S2352-8478(15)00029-5

DOI: 10.1016/j.jmat.2015.02.002

Reference: JMAT 15

To appear in: Journal of Materiomics

Received Date: 1 January 2015

Revised Date: 6 January 2015

Accepted Date: 20 February 2015

Please cite this article as: Fu J, Cao S, Yu J, Dual Z-scheme charge transfer in TiO2-Ag-Cu2O composite for enhanced photocatalytic hydrogen generation, Journal of Materiomics (2015), doi: 10.1016/j.jmat.2015.02.002.

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Graphical Abstract

H+/H2 0.0 V

02/H20+1.23V

Red^Oxd

A dual Z-scheme TiO2-Ag-Cu2O composite nanotube photocatalyst was prepared. The prepared photocatalyst exhibits high photocatalytic H2-production activity.

Original Article

Dual Z-scheme charge transfer in TiO2-Ag-Cu2O composite for enhanced

photocatalytic hydrogen generation

Junwei Fua, Shaowen Caoa'* and Jiaguo Yua,b'* aState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China ^Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

* Corresponding author. Fax: 0086-27-87879468; Tel.: 0086-27-87871029. E-mail: swcao@whut.edu.cn (S. Cao); jiaguoyu@yahoo.com (J. Yu)

Received date:2015-01-01; Revised date:2015-01-06; Accepted date:2015-02-20

Abstract: Photocatalytic hydrogen generation is one of the most promising solutions to convert solar power into green chemical energy. In this work, a multi-component TiO2-Ag-Cu2O composite was obtained through simple impregnation-calcination of Cu2O and subsequent photodeposition of Ag onto electrospun TiO2 nanotubes. The resulting TiO2-Ag-Cu2O photocatalyst exhibits excellent photocatalytic H2 evolution activity due to the synergetic effect of Ag and Cu2O on electrospun TiO2 nanotubes. A dual Z-scheme charge transfer pathway for photocatalytic reactions over TiO2-Ag-Cu2O composite was proposed and discussed. This work provides a prototype for designing Z-scheme photocatalyst with Ag as an electron mediator.

Keywords: Z-scheme, surface plasmon resonance (SPR), TiO2, photocatalysis, hydrogen generation

1. Introduction

The rapid consumption of non-renewable fossil fuels and the accompanying environmental pollution are forcing people to find clean and sustainable energy. Due to its high energy storage and environment friendliness, hydrogen is considered to be one of the best potential candidates. Since the electrochemical photolysis of water at TiO2 electrode was firstly reported by Fujishima and Honda in 1972 [1], numerous efforts have been made to improve the performance of TiO2 on photocatalytic hydrogen generation, which is mainly restricted by the fast recombination rate of photogenerated electron-hole pairs [2-5]. In recent years, hybrid or multi-component photocatalysts have shown significant advantages in suppressing the recombination of photogenerated electron-hole pairs through an efficient charge transfer process, and thus driving efficient photo-reduction and oxidation reactions at spatially separated sites [4-6]. Therefore, the construction of multi-component photocatalysts is recognized as an effective strategy to enhance the photocatalytic performance of TiO2.

Cu2O has been proved to be a good promoter to hybridize with TiO2 [7-17]. This is because (i) Cu2O with a narrow band gap of 2.0 eV can extend the absorption to visible-light range with wavelength up to 620 nm; (ii) both the conduction (CB) and valence bands (VB) of Cu2O lie higher than those of TiO2, which is favorable for the efficient transfer of excited electrons and holes between each other. Particularly, the excited electrons of Cu2O transfer to CB of TiO2 and the excited holes of TiO2 transfer to the VB of Cu2O, respectively. This process involves a typical charge transfer mechanism of a semiconductor heterojunction. The Cu2O/TiO2 heterojunctions have thus shown greatly enhanced activities as compared to single TiO2, due to the efficient charge separation between Cu2O and TiO2 [7,8,16]. However, it is known that the electrons in less negative CB and holes in less positive VB show weaker redox ability. Consequently, the resultant shortcoming of the typical semiconductor heterojunction is that the redox ability of transferred electrons and holes are reduced [5,16], which

negatively affect the photocatalytic reactions. Thus, developing photocatalytic systems with both fast electron-hole separation and strong redox ability is still a challenge.

A Z-scheme charge transfer mechanism that is different from the typical charge transfer mechanism of a semiconductor heterojunction, has also been proposed and investigated in the past decade. Such a Z-scheme pathway of charge transfer can preserve the oxidative holes in the lower VB and reductive electrons in the higher CB, resulting in not only greatly improved separation efficiency but also strong redox ability of photogenerated electrons and holes. Till now, a number of Z-scheme photocatalytic systems have been reported such as TiO2/CdS [18,19], ZnO/CdS [20,21], anatase/rutile [22], g-C3N/TiO2 [23], AgBr/Ag/AgI [24], H2WO4'H2O/Ag/AgCl [25], AgBr/Ag/BiOBr [26], Ag2CrO4-GO [27] and Bi20TiO32/Ag/AgCl [28], all of which have exhibited much better photocatalytic performance than the single-component photocatalyst. Nevertheless, in a semiconductor heterojunction, the Z-scheme charge transfer process usually faces the competition of the typical charge transfer process [20,29]. In this regard, a conductor or a contact interface with low contact resistance can be applied as an electron mediator to speed up the desirable specific carrier transfer [16,18,19,21,24-26]. For instance, Ag has been used as such mediator in many Z-scheme systems [19,24-27] because it has excellent electron conductivity. In addition, Ag can enhance the absorption of visible light and accelerate the electron transfer by a surface plasmon resonance (SPR)-induced local electric field, for example, anomalous Ag nanoparticles in AgI/Ag/AgBr composite [24], and 5-10 nm sized Ag nanoparticles in H2WO4'H2O/Ag/AgCl composite [25].

In this study, we introduce a dual Z-scheme TiO2-Ag-Cu2O photocatalytic system, in which Cu2O was loaded onto electrospun TiO2 nanotubes by a facile impregnation-calcination method, and Ag was subsequently deposited onto the photocatalyst through a photodeposition method. Under UV-vis light irradiation, both TiO2 and Cu2O can be excited. The photogenerated electrons in the CB of TiO2 will transfer to Ag due to the formation of Schottky barrier on the metal-semiconductor interface. Meanwhile, the SPR-induced local electric field will drive the electrons of Ag to combine with the holes

on the VB of Cu2O. Finally, electrons on the CB of Cu2O will react with H+ to generate H2. As a result, the TiO2-Ag-Cu2O composite shows greatly improved photocatalytic performance for hydrogen generation than TiO2, TiO2-Ag, and TiO2-Cu2O.

2. Experimental

2.1 Materials

All chemicals were of analytical grade and without further purification. Tetrabutyl titanate (Ti(OC4H9)4, TBOT) was purchased from Shanghai Kefeng Industry Co., Ltd. Polyvinylpyrollidone (PVP Mw = 1.3x106) was purchased from Aladdin Industrial Corporation. Paraffin liquid was purchased from Tianjin Guangcheng Chemical Reagent Co., Ltd. Ethanol (C2H5OH) and Copper (II) nitrate trihydrate (Cu(NO3)2.3H2O) were purchased from Sinopharm Chemical Regent Co., Ltd. Silver nitrate (AgNO3) was purchased from Tianjin Kemi'ou Chemical Reagent Co., Ltd. Methanol (CH3OH) was purchased from Tianjin Beichen Founder Reagent Factory. Deionized (DI) water was used in all experiments.

2.2 Preparation of TiO2 electrospun nanotubes

The TiO2 nanotubes were prepared by a coaxial electrospinning method modified from that described by Xia et al [30]. In a typical procedure for electrospinning, the precursor solution consisting of 1 g PVP, 5 mL TBOT, and 10 mL C2H5OH was vigorously stirred for 12 h. The solution was then added to a 20 mL syringe connected to the outer capillary of a coaxial two-capillary spinneret. The inner capillary of the two-capillary spinneret was connected to a 2.5 mL syringe filled with paraffin liquid. The diameters of the outer and inner capillaries were 1.6 mm and 0.7 mm, respectively. The feeding rate for PVP solution was set at 1 mL/h. For paraffin liquid, the feeding rate was 0.2 mL/h. Electrospinning was performed at 25 °C for 10 h with an applied potential of 15 kV and a distance of ~10 cm from the

tip to the collector. The relative humidity of the electrospinning chamber was kept at ~30%. The as-prepared electrospun nanofibers were kept in air for 2 h to allow the complete hydrolyzation of TBOT precursor. The resultant composite nanofibers consisting of amorphous TiO2, PVP and paraffin liquid were heated to 500 oC at a rate of 2 oC/min, and kept for 6 h to remove the PVP and paraffin thoroughly. Then the electrospun TiO2 nanotubes were obtained.

2.3 Preparation of TiO2-Ag-Cu2O photocatalyst

The TiO2-Ag-Cu2O photocatalyst was prepared from TiO2 electrospun nanotubes by loading Cu2O and Ag through impregnation-calcination and photodeposition methods, respectively. Typically, 0.3 g TiO2 nanotubes were immersed in a Cu(NO3)2 solution consisting of 4.5 mg Cu(NO3)2.3H2O and 20 mL deionized water. The mixed solution was stirred at 80 oC until the solvent was completely evaporated. The obtained powder was calcined at 350 oC for 4 h to yield Cu2O/TiO2 composite. The Cu2O loading on TiO2 was estimated to be ca. 0.5%(in mass). Then 0.3 g Cu2O/TiO2 powder and 2.4 mg of AgNO3 was added into 16 mL deionized water under magnetic stirring. The Ag loading on TiO2 was estimated to be ca. 0.5%(in mass). 4 mL methanol was injected into the above suspension, followed by the irradiation of a UV-vis light lamp for 2 h. After that, the suspension was collected and thoroughly rinsed with deionized water and ethanol three times to remove the residual impurity. This sample was denoted as TCA and the original electrospun TiO2 nanotube was named as T. The electrospun TiO2 nanotubes loading with just 1%(in mass) Ag and 1%(in mass) Cu2O were denoted as TA and TC, respectively.

2.4 Characterization

The X-ray diffraction (XRD) patterns were obtained on a D/Max-RB X-ray diffractometer (Rigaku, Japan) with Cu Ka radiation at a scan rate (29) of 0.05(o)/s. The average crystallite sizes were calculated according to the Scherrer equation using the full-width half-maximum data after correcting the

instrumental broadening. The morphology observation was performed on an S-4800 field emission scanning electron microscope (FESEM, Hitachi, Japan). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis were conducted with a JEM-2100F transmission electron microscope (JEOL, Japan), at an accelerating voltage of 200 kV. The UV-vis diffuse reflectance spectra were obtained on a UV-vis spectrometer (UV2550, Shimadzu, Japan); BaSO4 was used as a reflectance standard. X-ray photoelectron spectroscopy (XPS) measurement was done on a VG ESCALAB210 XPS system with Mg Ka source and all the binding energies were referenced to the C1s peak at 284.8 eV Raman spectra were recorded on a mirco-Raman spectrometer (Renishaw InVia) in the back-scattering geometry using a 514 nm Ar+ laser as excitation source. Photoluminescence (PL) emission spectra were taken with a fluorescence spectrophotometer (F-7000, Hitachi, Japan). The production of hydroxyl radicals (*OH) on the surface of the samples TC and TCA were detected by a photoluminescence method using terephthalic acid as a probe molecule [31]. 10 mg photocatalyst was added into 20 mL mixed solution consisting of 5*10-4 M terephthalic acid and 2*10-3 M NaOH. Under UV-vis light irradiation, the PL emission spectra of generated 2-hydroxyterephthalic acid were measured every 15 min using the excited wavelength of 325 nm. The actual amounts of Ag and Cu in the samples were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) using an Optima 4300 DV spectrometer (Perkin Elmer), and the results are shown in Table 1.

Table 1. ICP-AES results of samples

Sample No. ICP-AES (%, mass fraction)

TA 0.696 0

TC 0 0.657

TCA 0.342 0.327

2.5 Evaluation of photocatalytic activity

The photocatalytic reactions were carried out in a 100 mL Pyrex flask at ambient temperature and atmospheric pressure. The flask was sealed with silicone rubber septums. A 350 W xenon arc lamp was used as a UV-vis light source to trigger the photocatalytic reaction. The distance between the lamp and reactor was set as ~20 cm. In a typical photocatalytic H2-production experiment, 50 mg as-prepared photocatalyst was firstly suspended in 80 mL mixed aqueous solution containing 16 ml CH3OH and 64 ml H2O. Then the reactor was purged with nitrogen gas to ensure an anaerobic condition. After that, the gas-closed flask was irradiated and 0.4 mL gas sample was extracted from the flask at an interval of one hour. The H2 amount was analyzed by gas chromatography (GC-14C, Shimadzu, Japan, TCD, with nitrogen as a carrier gas and 5 A molecular sieve column). The apparent H2-production quantum efficiency (Qe) was measured under the above experimental conditions except different light source used. Four UV-LEDs (3 W, 365 nm, Shenzhen LAMPLIC Science Co. Ltd., China) positioned 1 cm away

from the reactor in four different directions were used as light sources to trigger the photocatalytic

reaction. The focused intensity and areas on the flask for each UV-LED was ca. 70 mW*cm- and 1 cm , respectively. The Qe was calculated according to the following Eq. 1.

N N X 2

Qe = -x100% (1)

Np Np W

Where, Ne is number of reacted electrons; Np is number of incident photons; Nm is number of evolved H2 molesules.

3. Results and discussion 3.1 Phase structure and morphology

Fig. 1. FESEM images of the sample T (a) and sample TCA (b); TEM (c) and HRTEM (d) images of the sample TCA.

Fig. 1(a) shows the typical FESEM image of the electrospun TiO2 nanotubes which are extracted from the sample T. It can be seen that these nanotubes are uniform with large aspect ratios. A clear tubular structure can be observed from the enlarged image shown in the inset. The average diameter and wall thickness of the nanotubes are ~750 nm and ~100 nm, respectively. Fig. 1(b) indicates that the nanotube feature of TiO2 does not change obviously in the sample TCA. It is noteworthy that there are a number of particles dispersed on the outer surface of the nanotubes, which should be ascribed to Ag and Cu2O nanoparticles. An in-depth observation was further performed by TEM, as shown in Fig. 1(c) and d. The hollow structure of a single nanotube is clearly displayed in Fig. 1(c). The HRTEM image of the selected area in Fig. 1(c) is presented in Fig. 1(d). The lattice fringe of 0.35 nm is consistent with (101)

planes of anatase TiO2, while the lattice fringe of 0.23 nm is assigned to the (111) planes of Ag. And the lattice fringe of 0.21 nm belongs to the (200) planes of Cu2O. These results confirm the presence of Ag and Cu2O on the surface of TiO2 nanotubes. Note that the connections between these components are intimate, and Ag particles mainly exist at the interface of Cu2O and TiO2. This is because the photogenerated electrons mainly assemble on the interface of Cu2O and TiO2 during the photodeposition process of Ag. Such unique structure is beneficial for the charge transfer among the three components. XRD was used to investigate the phase structure and crystallite sizes of the prepared samples. The XRD patterns of the samples T, TA, TC, and TCA are shown in Fig. 2. All of the peaks can be attributed to anatase (JCPDS, No. 21-1272) and rutile phases (JCPDS, No. 21-1276). The co-existence of anatase and rutile can enhance the separation efficient of photo-excited electrons and holes in terms of a mixed-phase heterojunction, which is favorable to improve the photocatalytic activity [2]. No peaks of copper oxide or silver can be observed, which is due to the low loading amount, small particle size and good dispersion [32]. The average crystallite sizes of anatase and rutile were calculated according to the Scherrer equation using the peaks of anatase (101) and rutile (110), respectively [33,34]. There is no obvious difference of the average crystallite sizes (12-13 nm) among all the samples. The results indicate that the crystal phases and sizes of the electrospun TiO2 nanotubes are stable during the loading process of copper oxide and silver.

A: Anatase R: Rutilc

2 Theta (degree)

Fig. 2. XRD patterns of the samples T, TA, TC and TCA.

3.2 XPS

XPS was used to identify the chemical status of the elements contained in the samples. The XPS survey spectra (not shown here) confirm the presence of Ag, Cu and Ag/Cu in the samples TA, TC and TCA, respectively. High-resolution XPS spectra of C 1s, O 1s, and Ti 2p for all samples show no obvious changes and thus are also not shown here. In detail, the spectra of C 1s show three peaks at 284.8, 286.0 and 288.9 eV, which are attributed to C-C (adventitious carbon), C-OH (hydroxyl carbon) and O-C=O (carboxyl carbon), respectively [33]. The spectra of O 1s show two peaks at 530.0 and 531.7 eV, which are related to Ti-O-Ti (lattice oxygen) and -OH (hydroxyl oxygen) [35]. The spectra of Ti 2p show two peaks at 458.9 and 464.6 eV, which belong to Ti 2p3/2 and Ti 2p1/2. The observed spin-orbit splitting between the Ti 2p3/2 and Ti 2p1/2 is 5.7 eV, in good agreement with the value of Ti4+ state in TiO2. Fig. 3(a) shows the high-resolution XPS spectra of Cu 2p. The two main peaks at 932.5 and 952.5 eV are attributed to Cu 2p3/2 and Cu 2p1/2, between which there is a weak peak that is the satellite peak of the Cu 2p3/2 [36,37]. Accordingly, the chemical state of Cu is most probably Cu+ or Cu0

in the samples TC and TCA [15,36,38]. It is known that Cu0 is difficult to exist during calcination; and the HRTEM image has also shown clear lattice fringes of Cu2O. Therefore, the element Cu should mainly exist as Cu+ in cuprous oxide (Cu2O) rather than Cu0. In the high-resolution XPS spectra of Ag 3d (Fig. 3(b)), the two peaks located at 368.3 and 374.3 eV can be assigned to metal Ag species, while the remaining two peaks at 367.7 and 373.7 eV are attributed to Ag+ which has not been reduced during the photo-reduction process [38].

955 950 945 940 935 930

Binding eneigy (eV)

i ■ i • i ■ i ■ i 1 i1

376 374 372 370 368 366

Binding eneigy (eV)

Fig. 3. High-resolution XPS spectra of Cu 2p (a) and Ag 3d (b) for the samples TA, TC and TCA.

3.3. UV-vis diffuse reflectance spectra

The comparison of UV-vis diffuse reflectance spectra of the samples is presented in Fig. 4. For all the samples, a significant increase in the absorption region at wavelength shorter than ~ 410 nm can be assigned to the intrinsic absorption of TiO2. In particular, the samples TA, TC and TCA exhibit an enhanced light absorption than the sample T in the range of 400-800 nm. This result suggests that loading Cu2O and Ag can enhance the visible-light absorption which may improve the photocatalytic activity. For the samples TA and TCA, it is noteworthy that there is a broad visible-light absorption band centered at ~500 nm. In order to get an in-depth investigation, the spectra subtracting the contribution of TiO2 are obtained and shown in Fig. 4(b). The spectrum of TC-T shows an absorption edge at ~620 nm which is in agreement with the intrinsic absorption of Cu2O, (2.0 eV) [39,40]. Furthermore, the broad absorption bands of the samples TA and TCA in visible-light region are ascribed to the SPR absorption of Ag nanoparticles [19,24-27,41-44]. In addition, the sample TA exhibits a stronger light absorption than sample TCA, which is consistent with the darker color of sample TA with more Ag particles.

Wavelength (nm)

—i—i—i—i—i—i—i—i—

400 500 600 700 800 Wavelength (nm)

Fig. 4. (a) UV-vis diffuse reflectance spectra of the samples T, TA, TC and TCA. (b) The spectra of TA-T, TC-T, and TCA-T obtained by subtracting the contribution of TiO2 in the samples TA, TC, TCA.

3.4. Raman analysis

Fig. 5 shows the Raman spectra of the samples T, TA, TC and TCA. The three obvious peaks at around 395 cm-1 (B1g), 512 cm-1 (A1g) and 637 cm-1 (Eg(2)) can be assigned to anatase [34,45], while the weak signal of rutile is overlapped by that of anatase. It can be found that the Raman signals of samples TA, TC and TCA have been enhanced in comparison with that of sample T, mainly due to the surface-enhanced Raman scattering (SERS). Up to now, there are two possible mechanisms to explain SERS. One is the electromagnetic enhancement caused by the SPR on the metal surface. The other one is the chemical enhancement mechanism including Chemical-Bonding Enhancement, Surface Complexes Resonance Enhancement, and Photo-Induced Charge-Transfer Enhancement (PICT) [46]. Actually, the enhancement of Raman signals by Ag/Cu2O nanostructure has been reported in a previous literature [47]. Since Fig. 4 has shown that the sample TA exhibits a clear SPR peak of Ag at ~500 nm, it can be inferred that the enhanced Raman signals of sample TA are attributed to the SPR-induced electromagnetic enhancement. While for sample TC, no SPR peak has been observed in the visible-light range. In this case, the PICT process may play an important role on the SERS. As for the sample TCA, the enhancement of TCA should be attributed to the synergetic effect of PICT process of Cu2O and the SPR-induced electromagnetic effect of Ag.

200 400 600 800 1000 1200

Raman shift (cm1)

Fig. 5. Raman spectra of the samples T, TA, TC and TCA.

3.5. Photoluminescence spectra

The PL emission spectra were used to reveal the separation efficiency of photogenerated electron-hole pairs and to understand the lifetime of these photogenerated charge carriers [32,48]. As shown in Fig. 6, all spectra show similar shapes with six main emission peaks attributed to PL signals of TiO2, appearing at ~398, 410, 451, 468, 483, and 493 nm, respectively. The two peaks at 398 and 410 nm can be assigned to the emission of band gap transition of anatase and rutile. The other four peaks can be ascribed to excitonic PL signals, originating from surface oxygen vacancies and defects of the samples [32]. As compared to sample T, a decrease in emission intensity is observed for sample TC, indicating enhanced separation efficiency of electron-hole pairs in the Cu2O/TiO2 composite. The sample TA shows the lowest emission intensity, which reveals that the addition of Ag is effective for separating photogenerated electron-hole pairs, due to the formation of a Schottky barrier at the interface

of silver and TiO2. Therefore, it is not surprising that the sample TCA shows lower emission intensity than sample TC because the photogenerated electrons of TiO2 can easily transfer to silver.

350 400 450 500 550 600

Wavelength (nm)

Fig. 6. Comparison of PL spectra of samples T, TA, TC and TCA.

3.6. Photocatalytic activity

Photocatalytic hydrogen generation on various samples was evaluated under the irradiation of a

350 W xenon arc lamp using methanol as a scavenger. Control experiments indicated that no noticeable

hydrogen production was detected in the absence of either irradiation or photocatalyst, suggesting that

hydrogen was produced via photocatalytic reactions. Fig. 7 shows the comparison of photocatalytic

hydrogen generation activity of the as-prepared samples. It can be clearly seen that the loading

components has a significant influence on the photocatalytic activity of TiO2. As compared to the

sample T, both samples TA and TC exhibit remarkably enhanced photocatalytic hydrogen production,

demonstrating that Ag and Cu2O can be used as co-catalysts to enhance the photocatalytic hydrogen

generation activity [8,49]. Importantly, the highest hydrogen production rate is achieved for the sample TCA, which is owing to the synergetic effect of Ag and Cu2O on the photocatalytic activity of TiO2. The corresponding apparent quantum efficiency of the sample TCA is 2.3% at 365 nm wavelength light irradiation.

Samples

Fig. 7. Photocatalytic hydrogen generation activity of the samples T, TA, TC and TCA.

It is well known that the separation efficiency of photogenerated electrons-holes plays an extremely important role in photocatalytic hydrogen generation. For example, in the previous report [7], Cu@Cu2O showed an enhanced effect on the photocatalytic activity of TiO2, due to the efficient transfer of photo-excited electrons and holes through an Ohmic nanojunction. The best photocatalytic activity of Cu2O/Cu/TiO2 exceeded that of pure TiO2 by about 12 times. In this work, the photocatalytic performance of the sample TCA exceeds that of the sample T by more than 62 times. In order to investigate the charge transfer mechanism, we have measured the formation rates of *OH on the samples TC and TCA in aqueous solutions under the irradiation of xenon lamp by the photoluminescence technique, using terephthalic acid as a probe molecule. Terephthalic acid is a poor fluorescent molecule that could react with *OH to form highly fluorescent 2-hydroxyterephthalic acid. It has been reported

that the potential of OH-/*OH couples is about +2.3 V (vs. NHE) [31], lying in between the VB of Cu2O and that of TiO2. As such, the holes on the VB of TiO2 can react with OH- to generate *OH, while the holes on the VB of Cu2O is incapable. Fig. 8(a) presents the changes of PL spectra observed during the irradiation on the sample TCA dispersed in terephthalic acid solution. It can be seen that the fluorescence intensity increases significantly with increasing irradiation time. Fig. 8(b) shows the plots of fluorescence intensity at 426 nm versus irradiation time for terephthalic acid in the presence of samples TC and TCA, respectively. The PL in the presence of sample TC shows very low intensity suggesting the low concentration of *OH. Here we assume that the photogenerated charge transfer in the TiO2/Cu2O composite follows the typical charge transfer mechanism of a semiconductor heterojunction. When the TiO2 and Cu2O are excited by irradiation, the electrons on the CB of Cu2O will transfer to that of TiO2; meanwhile, the holes on the VB of TiO2 will transfer to that of Cu2O. Consequently, the hole density in the VB of TiO2 is depleted while that in the VB of Cu2O is enhanced (Fig. 9(a)). Due to the less positive position of the VB of Cu2O, the holes on Cu2O do not have enough energy to oxidize OH-or H2O to form *OH. Hence the sample TC has low generation rate of *OH. Contrarily, the fluorescent intensity in the presence of sample TCA is much higher than that in the presence of sample TC. It means that the photogenerated holes of the sample TCA accumulate in the VB of TiO2, suggesting a different charge transfer mechanism.

On the basis of the above results, a possible photocatalytic mechanism of the sample TCA is proposed and illustrated in Fig. 9(b). Under UV-vis irradiation, both Cu2O and TiO2 are photoexcited to generate electrons and holes. Since equilibrium of the Fermi levels has been established at the interfaces of TiO2, Ag and CuO before light irradiation, and electrons on the CB of TiO2 will migrate to Ag after light irradiation. On the other hand, the strong SPR of Ag nanoparticles results in an enhanced local electric field around the interfaces, which can drive the electrons of Ag to inject into the VB of Cu2O to recombine with holes or into the CB of Cu2O via direct electron transfer or plasmon-induced resonant energy transfer [50]. As a result, the accumulated electrons in the CB of Cu2O with high potential

energy can reduce the H+ to produce H2, and the holes left behind the VB can achieve the photo-oxidation process. It is noteworthy that this type of charge transfer pathway retains the photogenerated holes on the more positive VB of TiO2 and electrons on the more negative CB of Cu2O, which not only greatly improve the separation of electron-hole pairs, but more importantly can preserve their high redox ability [29,51]. As this charge transfer pathway is analogous to that of the Z-scheme mechanism in a double "Z" shape, we hereby tentatively call it "dual Z-scheme charge transfer".

Wavelength (nm)

■1 «5

4J U S

£ o s

- * TC

/ / Htr TCA

10 20 30 40 50 Illumination time (min)

Fig. 8 (a) Changes of PL spectra with UV-vis irradiation time in the presence of TCA sample in a 5x10"4 mol/L basic solution of terephthalic acid (excitation at 325 nm); (b) plots of fluorescence intensity at 426 nm vs. irradiation time for terephthalic acid on samples TC and TCA.

H+/H, 0.0 V

02/H20+1.23V

Fig. 9 Schematic illustration for the charge transfer in sample TC (a) and sample TCA (b) under UV-vis light irradiation.

4. Conclusions

In summary, we construct a multi-component TiO2-Ag-Cu2O photocatalytic system based on the electrospun TiO2 nanotubes by a facile impregnation-calcination route combined with a photodeposition strategy. The TiO2-Ag-Cu2O composite exhibits efficient photocatalytic activity for hydrogen generation due to the synergetic effect of Cu2O and Ag. The introduction of Cu2O and Ag result in an enhanced visible-light absorption ability and moreover, the SPR-induced local electric field causes a dual Z-scheme charge transfer pathway in the composite, which enables both high separation efficiency and high redox ability of photogenerated electrons and holes. This work demonstrates that the SPR effect of Ag can adjust the transfer of photogenerated electron-hole pairs and provide a new insight into the design of highly efficient photocatalysts for hydrogen generation.

Acknowledgments

This work was supported by the 973 program (2013CB632402), and NSFC (51272199, 51320105001, 51372190, and 21433007). Also, this work was financially supported by the Natural Science Foundation of Hubei Province of China (2014CFB164), Deanship of Scientific Research (DSR) of King Abdulaziz University (90-130-35-HiCi), the Fundamental Research Funds for the Central Universities (WUT: 2014-VII-010, 2014-IV-058, 2014-IV-155), Self-determined and Innovative Research Funds of SKLWUT (2013-ZD-1), and a WUT Start-Up Grant.

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Jiaguo Yu received his BS and MS in chemistry from Central China Normal University and Xi'an Jiaotong University, respectively, and his PhD in Materials Science in 2000 from Wuhan University of Technology. In 2000, he became a Professor at Wuhan University of Technology. He was a postdoctoral fellow at the Chinese University of Hong Kong from 2001 to 2004, a visiting scientist from 2005 to 2006 at University of Bristol, a visiting scholar from 2007 to 2008 at University of Texas at Austin. His current research interests include semiconductor photocatalysis, photocatalytic solar fuel production and so on. See more details on: http://www.researcherid.com/rid/G-4317-2010.

nJunwei Fu obtained his B.S degree in materials science and engineering from Hubei University of Technology in 2012. From 2012 to present, he studies in Prof. Yu's group as a master candidate in Wuhan University of Technology. His current research focuses on the synthesis and properties of photocatalytic materials.